1. Technical Field
The present invention relates to high fidelity audio systems and, more particularly, high fidelity digital audio systems that require sampling rate modification.
2. Related Art
There exist different sampling rate conversion algorithms to alter the sampling rate of a signal from one sampling rate, f1, to another sampling rate, f2. Typically, it is essential to change between two different sampling rates without degrading any of the useful information in the original digital signal. Several techniques are used traditionally. The conceptually simplest method involves converting the digital signal sampled at the rate f1 into an analog signal using an accurate digital-to-analog converter (DAC). The resulting analog signal is lowpass filtered to remove digital images and then sampled at the rate f2 using an accurate analog-to-digital converter (ADC). While conceptually simple, the harmonics and noise distortion occurring in the conversion between the analog and digital domains degrades the quality of the resulting signal to a degree that renders the method unusable for high fidelity applications such as digital stereo audio.
A conventional multi-rate technique has been popularly employed for digital sampling rate conversion for a fixed ratio M/N, where M and N are positive integers. This method is only effective when M/N is quite large or small; for M/N≈1, a task referred to as “near-unity fractional sampling rate alteration”, the method demands excessive computational complexity. For example, a task of primary focus for the present invention is the conversion of digital audio between the sampling rates of f1=44.1 kHz and f2=48 kHz, a task commonly referred to as “compact disk (CD) to digital audio tape (DAT) conversion”, as shown in FIG. 1. Expressed as a ratio of integers, M/N=160/147, or, M/N≈1.088. The input signal is first up-sampled by a factor 160, then passed through a sharp digital lowpass filter, and finally decimated by a factor 147. Implementing a 160-fold interpolation for digital audio requires a computationally very complex digital algorithm and this method is thus undesirable for CD to DAT conversion.
What is needed, therefore, is a method and an apparatus that addresses the problem of efficiently altering the sampling rate of a high fidelity digital signal, such as digital audio, between two sampling rates that are only slightly different.